Tutorial: update Building a Graph Convolutional Network tutorial (#4060)
Tutorial: update Building a Graph Convolutional Network tutorial (#4060)

* update build_gcn.py tutorial

updates
* support bias in GCN layer
* download pretrained gcn model
* verify model accuracy
* use time_evaluator to measure runtime

* fix adding bias in gcn layer

* remove printing output

* fix small bug

* add DGL-PyTorch comparison into the build_gcn tutorial

* add accuracy testing

* adjust import order

* handle different dgl versions

* update number for dgl version checking

sxjscience/tvm,tqchen/tvm,tqchen/tvm,dmlc/tvm,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,sxjscience/tvm,sxjscience/tvm,sxjscience/tvm,tqchen/tvm,tqchen/tvm,sxjscience/tvm,sxjscience/tvm,tqchen/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,sxjscience/tvm,tqchen/tvm,tqchen/tvm,dmlc/tvm,sxjscience/tvm,dmlc/tvm,sxjscience/tvm,Laurawly/tvm-1,tqchen/tvm,dmlc/tvm,tqchen/tvm,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,tqchen/tvm